1. Field of the Invention
This invention relates to a component for coupling optical energy out of an optical fiber, and particularly to an optical fiber tap that efficiently couples optical energy out over a relatively broad wavelength range.
2. Description of the Prior Art
The ever-increasing complexity of fiber optic networks has created a need for devices that can measure the optical energy flowing through an optical fiber. Such devices are useful for network monitoring and control purposes, in many ways analogous to water gauges used to monitor the flow of water through pipes.
In order to measure the flow of optical energy in an optical fiber, a small fraction of the optical energy must be redirected out of the fiber and onto a detector. The detector converts the optical energy into an electrical signal that serves as a representation of the optical energy flowing in the fiber. A device commonly referred to as an “optical tap” performs the function of removing a fraction of optical energy from an optical fiber.
A variety of techniques for tapping light out of an optical fiber are well-known in the art. One such technique is described in U.S. patent application Ser. No. 10/390,398 (“the '398 application”), in which a CO2 laser beam is used to create a two-part structure in a fiber comprised of an annealed microbend and a reflecting surface formed in the cladding of the fiber for directing the light scattered by the microbend out of the side of the fiber.
While this technique offers many desirable features, such as small size, low insertion loss, and ease of manufacture, it suffers from inherent wavelength dependence owing to the guiding properties of the fiber.
One measure of this wavelength dependence is a variation with wavelength of tap efficiency. The tap efficiency is defined as a ratio of the optical power tapped out to the total optical power that is lost by introducing the tap into the optical path. Efficiency is reduced when light is scattered out of a fiber core but fails to exit the fiber at the tap and instead is lost at points downstream of the tap.
Unfortunately, in optical taps made in conventional telecommunications fiber according to the teachings of the '398 application, the tap efficiency may vary by as much as 400% over the wavelength band from 1310 nm to 1550 nm. While wavelength dependence may be of little, if any, concern for narrow-band applications, broadband applications require that the tap efficiency remain relatively constant over the 1310 nm to 1550 nm wavelength region.